A conventional method of heating hot water by solar energy is with a tank-over-collector thermosyphon system. A solar collector is located below a water storage tank. The collector is connected to the tank by pipes, one pipe connecting the top of the collector to the top of the tank and another pipe connecting the bottom of the collector to the bottom of the tank. During the day when solar radiation heats the water in the collector, the heated water rises because of its lower density, and enters the storage tank through the top pipe. The collector is replenished through the bottom pipe with cool water from the bottom of the tank. Thus, while the sun heats the collector, there is a thermosyphon circulation, with hot water continually flowing from collector to storage.
When the sun sets, the less dense heated water tends to remain at the top of the tank for use during the night, or even the following day. However, if the storage tank was not above the collector, at night a "reverse thermosyphon" circulation would occur. Solar-heated water in the storage tank would flow back into the collector where it would cool, and the cooled water would in turn flow back into the tank.
However, tank-over-collector systems, though not subject to reverse thermosyphon circulation, present a number of other problems. The tank is a heavy object, often weighing over 600 pounds. Building a structure strong enough to support the weight of the tank is expensive. In addition, such a structure is top-heavy and prone to topple in strong winds. Also, a solar collector is often less obstructed by trees if it is positioned on a roof, but positioning a storage tank higher than a collector on a roof is not only difficult, but lacks aesthetic appeal as well.
In contrast, a fairly compact unit can be built if the tank is not above the collector. For example, the tank can be positioned at same level as the collector in flat roof or ground installations, the tank providing a strong support member for the collector. But, as mentioned, such a system is very inefficient, because heat collected by day leaks away at night due to reverse thermosyphon circulation.